Two Discovery astronauts stepped into space Saturday, floating, crawling, drifting and dangling from gantries, handholds and tethers all over the space shuttle and the international space station.

At Mission Control, Wayne Hale, the deputy shuttle project manager, announced that NASA planners added an extra day to Discovery's mission to transfer equipment, water and other consumables to the space station, anticipating that a long time could elapse before the grounded shuttle fleet flies again.

The six-hour, 50-minute spacewalk was the first of three for Soichi Noguchi, a Japan Aerospace Exploration Agency astronaut, and Stephen Robinson, a guitar-playing engineer from California, and the first by any space shuttle crew in the 21/2 years since shuttle Columbia disintegrated on reentry.

The pair used the time to test new repair techniques, install part of a storage platform, and replace and fix broken equipment that had crippled the space station for months. They accomplished all of their tasks in the allotted time and even added a couple of extras at the end.

"They did it like they could do it in their sleep," said a beaming Cindy Begley, Mission Control's spacewalk chief and the pair's chief mentor for more than three years. "I'm more than happy at the way everything went."

So was Mark Ferring, the space station flight director in Houston, who finished the day with a new gyroscope to hold the station in place and a new Global Positioning System antenna. "It's really nice to have the backup," Ferring said.

The crew of the space station, which depends on the shuttle for heavy equipment and large-scale resupply, had been watching the machinery wear out and its resources dwindle ever since the Columbia disaster.

Discovery's mission, in the first shuttle flight since 2003, was to inaugurate a new era in shuttle travel, but the orbiter's external fuel tank lost a 0.9-pound piece of foam insulation during launch, prompting NASA to halt all shuttle flights until the problem is fixed, a decision that put September's scheduled mission in doubt. A similar but larger chunk of debris doomed Columbia.

Despite the cloud hanging over the shuttle program, Discovery's mission has proceeded virtually without flaw since liftoff Tuesday. The orbiter appeared to have emerged unscathed from the launch, and a new suite of cameras and other imaging devices provided engineers with unprecedented images of the shuttle's heat shielding.

Shuttle astronauts used a new 50-foot boom sensor to conduct laser imaging of several "areas of interest" where scarring might have occurred on the "reinforced carbon-carbon" leading edge of Discovery's wings.

Mission operations representative Philip L. Engelauf said at a Johnson Space Center news conference that the latest survey appeared to have found nothing significant, but he suggested that analysts may "develop another shopping list of targets."

Noguchi cracked open Discovery's airlock hatch and stepped into the void at 5:46 a.m. Eastern time, in time for sunset over Southeast Asia. "What a view!" he exclaimed.

Despite a long and varied list of jobs, attention on the ground focused initially on tests of two experimental techniques that NASA would like to use for the onboard repair of launch-related damage to a shuttle's heat shielding.

Early reviews were somewhat mixed. Robinson used a black, glue-like compound that he described as "licorice-flavored pizza dough" to fill cracks and gouges in pre-damaged panels of the reinforced carbon-carbon.

The compound is formally known as nonoxide adhesive experimental, or NOAX. Robinson applied it with a caulk gun, then forced it into the cracks and holes with a putty knife.

This process appeared to work well, and the NOAX filled the damaged spots easily and without extensive bubbling. But it swelled up, even after it started to dry. "It's seems almost elastic, like it was a little rubbery," Robinson said at one point. "And, once it starts to dry, you can't clean the spatula."

Begley said engineers had anticipated the swelling, "and as long as it's not too high, we're doing great." Robinson overcame the dirty spatulas by swapping them for clean ones offered by Noguchi.

Noguchi had no trouble with an "emittance wash" used on chipped sections of thermal protection tiles, daubing the damaged areas with a spongy, shoe polish-like applicator, then spreading the liquid with a stubby paintbrush.

The spacewalkers' bulky suits require that hand tools be minimally complicated so that sausage-size fingers can manipulate them easily. Noguchi and Robinson, bent over the tiles' carrying case as they handed the spatula and the caulk gun back and forth, looked like a pair of snowmen performing surgery.

And it went slowly at first. "Space is different," Begley said. "At first, you feel vertigo. You feel like the world is moving around you."

But as the day wore on, the spacewalkers moved with much more confidence. They have trained together since 2001.

"How cool is this?" remarked Robinson as the station's crane picked him up and dangled him upside down as he moved to install the mounting bracket for a storage platform to be put in place during the team's final spacewalk later this week.

Astronaut Andrew Thomas, choreographing the spacewalk from inside Discovery, offered Robinson a bit of advice. "Thanks for the heads-up," Robinson said. "I mean the heads-down."

Using handholds, fabric tethers, foot restraints and the crane, the walkers scrambled all over the space station. Robinson rewired a gyro that had short-circuited in March. Noguchi replaced a GPS antenna. Robinson saved enough time so he could recover a pair of outdoor experiments. Noguchi took pictures of thermal protection blankets that appeared to have come loose near one of the shuttle's windows.

In the end, however, the space station's Ferring may have been the spacewalk's biggest beneficiary. With his new GPS antenna in place, his gyro running easily again and a second seized-up gyro set to be replaced Monday, the space station is now in better running shape than it has been for a long time.

And with the extra day, Ferring was looking forward to "stealing as many laptops as we can" from the shuttle, maybe picking up an extra spacesuit and getting rid of 21/2 years worth of worn out equipment and trash. "I'm having a ball," he said.

Astronaut Stephen Robinson approaches the docking port hatch in Discovery's payload bay in this view from the helmet camera of Japanese astronaut Soichi Noguchi after their spacewalk, the first of three they are scheduled to take during the current mission.